1. Field of the Invention
The present invention relates generally to snowplowable pavement marker construction and, more particularly, to pavement markers which impart reduced impact forces when struck by a plow blade during a snowplowing operation.
2. Description of the Related Art
Pavement markers have long been widely accepted as permanently installed devices for providing visible signals which delineate traffic lanes of highways and control the flow of traffic in conjunction with or in place of painted traffic lines. A large number of such markers employ reflectors which retroreflect light emanating from oncoming vehicle headlights to provide a visible signal to operators of such vehicles.
It is also common that in regions where frequent snow fall is experienced the reflectors are protected from snowplow damage by metal frame devices which hold the reflectors firmly to the pavement and serve to deflect a snowplow blade thus preventing the blade from stripping or breaking the reflectors. Such devices are disclosed, for example, in Heenan U.S. Pat. No. 4,195,945, Flanagan U.S. Pat. No. 5,257,875 and Flanagan et al. U.S. Pat. No. 5,277,513, all which are commonly assigned to the assignee herein. In the '945 patent, for example, snowplowable pavement markers are disclosed which include a metal frame, or base member, having two arcuate bottom keel members interconnected by an arcuate bottom support member. The upper surfaces of the keel members define inclined ramps from a plane at one end of the base member toward the other end thereof to corresponding coplanar top surfaces. The support member has a support surface lying below the plane for supporting thereon a retroreflector which is partially recessed below the plane. The keel members and the support member are secured in complementary arcuate recesses in the pavement with the plane of the retroreflector support surface being slightly below the pavement surface.
In designing these prior art snowplowable pavement markers it has been a continuing goal to reduce the overall height of the marker frame member to a minimum level above the pavement surface without undesirable reduction of the retroreflectance of the marker and, accordingly, the visibility of the marker. In this regard it has heretofore been recognized that the angle between the pavement surface and the inclined ramps of the frame members should be minimized to reduce impact of a snowplow bade on that portion of the frame member projecting about the pavement surface. This is so because not only can such impact damage or destroy the frame member it can cause significant jolting of the snowplow vehicle during a plowing operation. However, although the ramp angle theoretically could be reduced as low as desired, a lower ramp angle generally requires a longer ramp and thus a longer frame member to maintain the same reflector height above the pavement surface. Not only would a longer frame be heavier and more costly, it would require specialized equipment to install. Also, current frame member designs typically use either a series of ramp slopes to guide the blade over the reflector or, in some cases, these slopes are rounded off into a convex shape. A disadvantage of these designs is that they produce large strike forces when plowing is performed at pavement level.
Another goal in the design of snowplowable pavement markers has been to design such markers in which the retroreflector members are adequately protected from snowplow blade damage when the blade is set at a large angle relative to the direction of vehicle travel. Generally, common retroreflector members are made of plastic materials which can easily be damaged by a heavy metal plow blade. To this end, the aforementioned '875 patent discloses a type of pavement marker specifically designed to protect the reflector against the action of a snowplow blade set at an angle of up to sixty degrees from a line extending perpendicular to the direction of travel of the snowplow vehicle. This is particularly advantageous where the tip of a snowplow blade, set at such a large angle, could intrude between the pavement marker ramps and contact the reflector. Such larger angles typically are desired in extremely heavy snow areas where high speed plowing is common, such as on interstate roads. The '875 device, which is a one-way plowable device, has two spaced-apart primary ramp members joined by a body portion. An auxiliary ramp member is centrally positioned on the front of the body portion between the primary ramp members and serves to support a snowplow bade when the blade tip is located between the two primary ramp members.
Although the aforementioned pavement markers have gained considerable commercial acceptance and are now in widespread use, it is desirable to provide a snowplowable pavement marker that is of a standard length yet exhibits reduced effect of impact loads when struck by snowplow blades and is sufficiently high above the pavement as to allow for the use of a reflector which can be readily seen by an oncoming motorist. It is further desirable to provide such a pavement marker which is capable of deflecting a snowplow blade that is set at a relatively large angle relative to its direction of travel thus preventing possible damage to the marker's reflector. Still further it is desirable to provide such a pavement marker which is readily constructed by known manufacturing techniques and is cost-effective to produce.